The Berkeley study found that the most heavily exposed children scored an average of 7 points lower on IQ tests compared with children with the lowest pesticide exposures, lead author Brenda Eskenazi. says.

On IQ tests, the average score is around 100.

Even a difference of 2 or 3 points — the size of the IQ loss caused by lead, which is known to cause brain damage — can have an enormous impact, says pediatrician Aaron Bernstein of Children's Hospital Boston.

That's because a population's IQ scores, when plotted on a graph, tend to fall along a bell-shaped curve. Shifting the entire curve down, even if just by a few points, causes a big jump in the number of kids with low intelligence and a dramatic loss in the number of super-smart ones, says Bernstein, who wasn't involved in the study. That can sharply increase the number of kids needing remedial education, says Bruce Lanphear, a professor at Simon Fraser University in Vancouver, Canada, also not involved in the study. …